Ceramic paint compositions are generally known to those ordinarily skilled in the art of applying ceramic paints to automotive and architectural glazings. Such ceramic paints are used, for example, to form opaque borders around the peripheral marginal surfaces of glazings used as windshields, sidelites, and backlites of motor vehicles.
Ceramic paints or pastes generally contain a mixture of metal oxides which together act as a coloring agent. The metal oxides are non-reactive with one another, as well as non-reactive with other compounds contained in the ceramic paint or the glass to which the ceramic paint is applied, at temperatures up to about 1,300.degree. F. The mixture of metal oxides may be adjusted so as to achieve a specific color for the ultimately produced fired ceramic paint field. For example, the opaque peripheral marginal paint bands fired onto automotive glazings are generally black.
Additionally, the ceramic paints generally known in the art contain one or more low melting glass frits, which melt at temperatures below 1,300.degree. F. These low melting glass frits are the materials which ultimately fuse the ceramic paint together, and to the glass sheet, to insure that the ceramic paint remains affixed to the glass sheet after being cooled to room temperature.
A vehicle is also generally mixed with the metal oxides and glass frit, to allow the ceramic paint to be applied to the surface of a glass sheet by conventional paint application processes. Generally, organic vehicles, e.g., pine oil, mineral oils, low molecular weight petroleum fractions, and the like, are used to allow the ceramic paint to be applied to the glass surface by brushing, spreading, or screen printing.
The ceramic paint compositions generally known in the art are, however, susceptible to scratching when applied to the surface of a glass sheet which is thereafter formed in a gas hearth furnace. The ceramic paint is applied to that surface of the glass sheet which is formed into a concave configuration by advancing over increasingly curved gas hearth forming blocks while the glass sheet rides on a cushion of hot gasses which elevate and maintain the glass sheet above its softening temperature. The distal portions of the glass sheet often unintentionally slidingly engage the surface of the gas hearth forming blocks as the glass sheet advances through the gas hearth furnace. Where those distal portions of the glass sheet have been previously coated with a ceramic paint, the paint is scratched. Thus, automotive and architectural glazings, which have been coated with a ceramic paint and thereafter formed in a gas hearth furnace, often have permanent unsightly surface abrasions on their painted portions.
U.S. Pat. No. 3,361,522 to Ritter, Jr. discloses a method for bending horizontally conveyed glass sheets between opposed shaping molds. To prevent marring of the glass surfaces, the press bending molds are covered with a relatively soft, non-abrasive, heat resistant material such as, for example, asbestos or fiberglass cloth. The patent does not suggest that the use of asbestos or fiberglass would aid in preventing the scratching of painted surfaces on glass sheets which are formed between press bending molds or formed by the gas hearth process.
U.S. Pat. No. 4,274,857 to Wolfe discloses a method for treating the engagement surfaces of press bending molds, to reduce the likelihood of marring the ceramic painted portions of a glass sheet, and to facilitate the removal of the engagement surfaces of the press bending molds from the painted portions of the glass sheet. A boron nitride compound is sprayed or brushed onto the fiberglass covers of the engagement surfaces of the press bending molds. This patent does not suggest the use of a ceramic paint adjuvant to eliminate scratches in the painted surfaces of glass sheets formed by the gas hearth process.
U.S. Pat. No. 4,432,782 to Seymour discloses a method and apparatus for shaping and conveying glass sheets in a gas hearth process, wherein certain surfaces within the furnace are coated with various materials that do not mar glass sheets which come into sliding contact therewith. The preferred coating material is boron nitride, although graphite, magnesium oxide, and titanium oxide are also disclosed. The patent does not suggest that ceramic painted surfaces of the glass sheets would similarly be protected from marring. Nor does the patent disclose a ceramic paint additive to prevent the formation of scratches which would otherwise result form such sliding contact.
U.S. Pat. No. 4,828,596 to Reinherz discloses glass enamel coating compositions, comprising a glass frit, a vehicle, and either copper sulfide or copper sulfate which acts as an anti-stick agent for separating the engagement surfaces of press bending molds away from the painted surfaces of the glass sheets during the forming operation. The patent also discloses that certain refractory materials such as zirconium oxide may be added to the glass enamel coating compositions to increase abrasion resistance. The patent does not suggest that adjuvants other than refractory materials may be added to a ceramic paint to prevent the formation of scratches on the painted surfaces of glass sheets formed by the gas hearth process, nor that the disclosed refractory materials would provide abrasion resistance apart from their combination with copper sulfide or copper sulfate.
Finally, U.S. Pat. No. 4,684,389 to Boaz discloses a method for forming a glass sheet having ceramic paint thereon, wherein finely divided zinc metal powder is included in the paint composition to prevent sticking of the ceramic paint to the engagement surfaces of press bending molds. The patent does not suggest that such a ceramic paint composition would be effective to prevent the formation of scratches in the painted portions of glass sheets formed by the gas hearth process. The patent likewise does not suggest that ceramic paint compositions containing zinc metal powder would be useful in any glass sheet forming processes other than ones such as press bending wherein the forming surfaces are intentionally brought into engagement with the painted surfaces of the glass sheet during the forming operation.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is rot to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention, nor that the cited prior art when considered in combination suggests the present invention absent the teachings herein.
It would be desirable to form ceramic painted glass sheets by the gas heart process, wherein the painted portions of the ultimately produced formed glass sheets would be free from surface scratches.